The invention relates to a method for commutating from a reverse-conducting IGBT operated in the diode mode to a reverse-conducting IGBT operated in the IGBT mode which form a power converter phase and are electrically connected in parallel with a direct-current voltage source. The invention further relates to an apparatus for performing the inventive method.
IGBTs that are able to conduct current in the opposite direction are also known as Reverse-Conducting IGBTs (RC-IGBTs). Said RC-IGBTs are a further development of the known reverse-blocking IGBTs. An RC-IGBT differs from a conventional IGBT in that the diode function and the IGBT function are combined in one chip. This results in a power semiconductor in which anode efficiency in the diode mode is dependent on the gate voltage. This demands a change in the way such devices are driven compared with conventional IGBTs.
In reverse-conducting IGBTs anode efficiency in the diode mode can be controlled by means of the gate. If the gate is turned on, anode efficiency is reduced, whereas the forward voltage increases and the stored charge decreases. If on the other hand the gate is turned off, anode efficiency remains high, as a result of which the forward voltage is low and the stored charge high.
This behavior of the reverse-conducting IGBT can be used in order to reduce the reverse-recovery losses of the reverse-conducting IGBT operated in the diode mode and the turn-on losses of the second reverse-conducting IGBT of a power converter phase.
A method for commutating from a reverse-conducting IGBT operated in the diode mode to a reverse-conducting IGBT operated in the IGBT mode is described in the publication “A High Current 3300V Module Employing Reverse Conducting IGBTs Setting a New Benchmark in Output Power Capability” by M. Rahimo, U. Schlapbach, A. Kopta, J. Vobecky, D. Schneider and A. Baschnagel, printed in ISPSD 2008. According to said known method, an IGBT operated in the diode mode is turned on after a predetermined first delay time, starting from the time instant of a setpoint turn-off control signal, has elapsed. Of the two RC-IGBTs connected in series, the IGBT operated in the IGBT mode is turned on after a predetermined second delay time, starting from the time instant of a setpoint turn-on control signal, has elapsed. Immediately before the RC-IGBT operated in the IGBT mode is turned on, the RC-IGBT operated in the diode mode is turned off again. For that purpose a time period is predefined for the RC-IGBT operated in the diode mode, for the duration of which said reverse-conducting IGBT remains turned on.
A shortcoming of said known method is the sensitivity toward poorly toleranced operating times. On the one hand the gate voltage of the reverse-conducting IGBT operated in the diode mode must be reduced below a so-called threshold voltage before a reverse current peak of the reverse-conducting IGBT operated in the diode mode is reached. On the other hand the reverse-conducting IGBT operated in the diode mode must not yet have been turned off for a long time when the reverse-conducting IGBT operated in the IGBT mode is turned on, because otherwise the effect of the decrease in anode efficiency is no longer active. However, the signal paths from a higher-ranking control device, for example a control device of a power converter, to the control circuits (also referred to as driver circuits) of the two RC-IGBTs electrically connected in series have a potential separation in each case. This leads to relatively wide tolerances in the switching times, thereby further widening the tolerances in the drive paths of the RC-IGBTs electrically connected in series. As a result the activation of the mutually coordinated delay times necessitates a great investment of time and resources.
The object underlying the invention is therefore to develop the known method in such a way that it is less sensitive to poorly toleranced operating times.